PHYSIOLOGIA PLANTARUM 74: 593-601. Copenhagen 1988 Carbon and nitrogen partitioning in young nodulated pea (wild type and nitrate reductase-deflcient mutant) plants exposed to NH4NO3 David C. Taylor, Barry J. Shelp, Louise M. Neison and Bernard Grodzinski Taylor, D. C , Shelp, B. J., Nelson. L. M. and Grodzinski. B. 1988. Carbon and nitrogen partitioning in young nodulated pea (wild type and nitrate reductase-defi- cient mutant) plants exposed to NHjNO,. - Physiol. Plant. 74: 593-601. Carbon and nitrogen partitioning was examined in a wild type and a nitrate reduc- tase-defieient mutant (A317) of Pisum sativum L. (cv. Juneau). effectively inoeulated with two strains of Rhizobium leguminosurum (I28C23 and I28C.S4) and grown hydroponically in medium without nitrogen for 21 days, followed by a further 7 days in medium without and with 5 niM NH4NO,. In wild-type symbioses the application of NH4NO, significantly reduced nodule growth, nitrogenase (EC 1.7.99.2) activity, nodule carbohydrates (soluble sugars and starch) and allocation of |"C]-labelled photosynthate to nodules, and increased the concentrations of nitrogenous solutes (NO,, NH;, amino acids) in roots. In nodules, there was a decline in amino acids together with an increase in inorganic nitrogen concentration. In contrast, symbioses involving A3I7 exhibited no change in nitrogenase activity or nodule carbohydrates, and the concentrations of all nitrogenous solutes measured (including asparagine) in roots and nodules were enhanced. Photosynthate allocation to the nodule was re- duced in the 128C23 symbiosis. Nitrite accumulation was not detected in any case. These data cannot be wholly explained by either the carbohydrate deprivation hypothesis or the nitrite hypothesis for the inhibition of symbiotic nitrogen fixation by combined nitrogen. Our results with A317 also provided evidence against the hy- pothesis that NO, and NHj or its assimilation products exert a direct effect on nitrogenase activity. It is concluded that more than one legume host and Rhizohium strain must be studied before generalizations about Rhizobium/lcgumc interactions are made. _^ Key words - Carbon and nitrogen partitioning, nitrate reductase, nitrogenase, pea plants, Pisum sativum, Rhizobium leguminosarum. D. C. Taylor, B. J. Shelp (corresponding author) and B. Grodzinski, DepI of Horti- cultural Science, Univ. of Guelph, (iuelph, Ontario. Canada NKl 2WI; and L. M. Nelson, National Research Councit of Canada. Plant Biotechnology Inst., Saskatoon, Saskatchewan, Canada S7N 0W9. Introduction ^j^^ ^^ bacterial respiration (Becana and Sprent 1987, The meehanisms responsihlo lor the inhibition of sym- Nelson and Edie 1988, Silsbury et al. 1986). Several biotic nitrogen fixation in legumes by combined N are researchers have determined in a variety of legumes the uncertain. However, nitrogenase inhibition is generally concentrations of carbohydrates (soluble sugars and interpreted as resulting either from a direct effect of starch) (Nelson and Edie 1988, Streeter 1986, Walsh et NO, and its assimilation products (NO,, NH|, amino al. 1987, Wasfi et al. 1986) and nitrogenous solutes acids) or indirectly from a reduction in energy supply (Nelson and Edie 1988, Silbury et al. 1986), as well as caused by diversion of carbohydrates away from the photosynthate partitioning (Latimore et al. 1977, Small nodules to sites of NO^ assimilation or by NO, ox- and Leonard 1969). Few studies, however, have deter- idation of leghemoglobin and consequently O, limlta- mined all these parameters in a single species, produc- Received 23 August, 1988 Physiol Plant 74. IMSX 593